INTRODUCTION Almost one-third of college students report recent cigarette use, primarily as "social smoking," and often in conjunction with alcohol use. While city-wide indoor smoking bans effectively reduce the number of social opportunities to smoke (eg, bars and music clubs), little is known about how these bans may impact the smoking behaviors of college students. Furthermore, nothing is known about how indoor smoking bans may impact students' drinking behaviors. The current study aims to determine the impact of a city-wide comprehensive indoor smoking ban on smoking and alcohol behaviors among a longitudinal sample of emerging adults. METHODS Data are from a 6-year longitudinal study (10 waves of data collection) that began the summer before college enrollment. Participants (N = 2244; 60% female) reported on their past 3-month smoking and drinking behaviors using Internet-based surveys at each wave. Piecewise linear growth modeling was used to determine how a city-wide comprehensive indoor smoking ban (implemented in the Fall of 2005 between Waves 4 and 5) impacted smoking frequency, cigarette quantity, drinking frequency, and number of binge drinking episodes. RESULTS Smoking and alcohol use increased from the summer before college through the semester before implementation of the city-wide smoking ban. While smoking frequency (P < .001) and cigarette quantity (P < .05) declined after the ban, drinking frequency increased (P < .001) and the number of binge drinking episodes remained stable. CONCLUSIONS Current findings suggest that comprehensive indoor smoking bans can influence the smoking behaviors of emerging adults, whereas trajectories of drinking are relatively unchanged.